The present invention concerns a process for the preparation of coarse crystalline alumina from aluminum hydroxide, which after preparation in a known manner, is used advantageously for abrasives, lapping and polishing and for refractories.
In the abrasives industry, a large demand exists for single crystal corundum particles in the grain size range 800 to 200 according to FEPA corresponding to 10-70.mu. average crystallite size.
The possibility has frequently been referred to of preparing the desired single crystal corundum directly from alumina production and thereby bypassing the technically difficult and economically undesirable process of melting and sintering. However, all previous economically justifiable efforts failed to reach an adequate crystal size and/or thin-plated shape.
The present invention overcomes these problems. The alumina produced in accordance with the invention consists of thick plates of corundum single crystals limited by their natural surfaces in the above mentioned particle size range. Also, the alumina produced in accordance with the present invention can be produced cost-effectively.
Alumina is produced on a commercial scale by the well-known Bayer process. After dissolution of the bauxite in sodium hydroxide liquor and subsequent precipitation, aluminum hydroxide is obtained in agglomerate form up to 100.mu. size. By calcination in rotating or fluidized-bed ovens, the aluminum hydroxide is converted to alumina. In this way, complete conversion to .alpha.-alumina is sought which is certainly achieved at temperatures of about 1200.degree. C. and over. An alumina prepared in this way has very small particles, seldom over 5.mu. crystal size, and for the above mentioned purpose e.g. polishing, is of little or no value.
It is not for lack of experimentation, especially directed producing abrasives and lapping properties, that alumina products from the Bayer process have not succeeded. It is known that the addition in small quantities of so-called calcination promoters or mineralizers, accelerated and/or decreased the phase change temperature to .alpha.-alumina. Simultaneously, there is obtained a shift of the crystallite particle size distribution to larger particles.
Effective compounds in this respect are especially the fluorides NaF, CaF.sub.2, AlF.sub.3 and N.sub.3 AlF.sub.6. DE-AS No. 11 59 418 teaches that a few tenths of a percent of hydrogen fluoride in the oven atmosphere has the same effect.
According to the flow or the heating rate and the kind and quantity of fluoride compounds, the transformation temperature to alumina and associated crystal size can be varied in a limited range.
According to DE-AS No. 28 50 064, the crystallite size is also dependent on the agglomerate size of the aluminum hydroxide. By repeated crystallization using seed crystals consisting of previously calcined product, fluoride containing additives and aluminum hydroxide agglomerates &gt;60 .mu., corundum crystals, .alpha.-Al.sub.2 O.sub.3 can be prepared with diameter D as the largest dimension perpendicular to the c-axis from 16 to maximum 250.mu., with diameter/height ratio D/H between 3 and 7, where the height H refers to the largest dimension parallel to the c-axis.
In processes which do not use seed crystals, only .alpha.-Al.sub.2 O.sub.3 crystals with the extremely thin, plate-like hexagonal shapes are obtained. The crystals have in the best case a diameter D of 25.mu., the majority of which are about 10.mu.. The height H in special cases is about a quarter of the diameter and for over 80% of the crystals ranges between one-sixth and one-tenth of the diameter D.
The disadvantage of crystals of this kind for abrasives, lapping and polishing purposes is the too small crystallite size and/or especially in the high diameter to height ratio. Platelike corundum crystals less than a diameter of 10.mu. are rarely used by the surface treating industries. Crystals with larger diameters but higher D/H ratios disintegrate very easily during use as an abrasive, especially with lapping and polishing, and then form cutting edges of random geometry. The presumed advantage of single crystal grains with all crystals of equal cutting geometry due to the natural surfaces and the high specific fraction of cutting edges, is at least partly reduced.
For this reason, alumina products from calcination of aluminum hydroxide have not previously made the expected applications in surface treating techniques.